1. Field of the Invention
The present invention relates to a solar cell and a method of manufacturing the solar cell, and more particularly, to a fibril wet solar cell containing carbon nanotubes and having low manufacturing costs, high photo-conversion efficiency, and a good formability, and a method of manufacturing the solar cell.
2. Description of the Related Art
As global concerns about polluting energy sources increase, interest in alternative energy sources to fossil fuels, such as coal or petroleum, has increased. For example, a solar cell is a device for converting solar energy into electrical energy. Photovoltaic energy conversion is achieved by exposing the solar cell (p-n junction diode) to photons to generate negative electrons and positive holes, and transferring the electrons and the holes to electrodes to generate an electromotive force. Such a solar cell is generally classified as a silicon solar cell or a compound semiconductor solar cell, according to the constituting material thereof.
A single crystalline silicon solar cell is mainly used as a silicon solar cell, and is generally referred to as a dry solar cell. The single-crystalline silicon solar cell has a major advantage in that it can be made thin. However, since the single-crystalline silicon solar cell is very expensive, it is utilized only for special purposes, such as the aerospace industry. A relatively inexpensive amorphous silicon solar cell is available for use, but has a low photo-converting efficiency.
A compound semiconductor solar cell comprising CuInSe2, CdTe, GaAs, and derivatives thereof has relatively good characteristics, but its use is very limited because of high cost, low efficiency, and low stability.
Currently, much consideration is being made for wet solar cells, which are low cost, environmentally-friendly, easily produced, and stable.
A wet solar cell consists of a semiconductor electrode and an electrolyte. A typical example of the wet solar cell is a combined solar cell having a crystalline TiO2 electrode composed of an n-type semiconductor and a Pt electrode. If light is radiated onto a surface of the crystalline TiO2 of the wet solar cell, electrons are excited, and transferred to a conduction band. When the electrons reach the Pt electrode through a lead wire, they react with protons to generate hydrogen. According to a principle of a typical wet solar cell, holes in a valance band deprive water molecules on the surface of TiO2 of electrons. The holes disappear to generate oxygen. Instead of dissolving the water, the wet solar cell generates electrical energy via a resistor of an external circuit.
When the wet solar cell composed of the semiconductors absorbs light energy having a wavelength shorter than a desired level (i.e., having more than a band gap energy, Eg), carriers are increased, thereby generating an electric current. However, the wet solar cell cannot use light energy lower than Eg. Therefore, the wet solar cell composed of TiO2, of which the band gap energy is 3.2 eV, can utilize only up to 4% of sunlight. Consequently, the efficiency of such a wet solar cell is very low.
In order to solve this problem, a wet solar cell having a band gap energy lower than that of TiO2 to improve light absorbing efficiency of visible rays has been proposed. A desired dye capable of absorbing the visible rays is coated onto a surface of the semiconductor, and light of a wavelength to be absorbed by the dye is radiated onto the surface, such that the number of carriers in the semiconductor is increased. Such a wet solar cell is referred to as a dye-sensitized solar cell or Gratzell cell.
The dye-sensitized solar cell was first developed by Gratzell in Switzerland and used a dye sensitizer capable of absorbing visible light coated on TiO2, which is a typical n-type semiconductor. A photosensitive dye of the n-type semiconductor absorbs photons to excite electrons, and the electrons circulate through an electric circuit via the n-type semiconductor. A redox substance receives the electrons from an opposite pole, and electrons of the photosensitive dye are returned to a ground state, so that the electrons are excited again.
There is currently some research on the dye-sensitized solar cell or Gratzell cell. Examples of the dye-sensitized solar cell are included in Nature vol. 395, pp. 583-585 (8 Oct. 1998), U.S. Pat. Nos. 5,350,644, 5,441,827, and 5,728,487, and European Patent No. EP0886804A.
These patents disclose dye-sensitized solar cells and their method of manufacture. The solar cell includes an electrode coated with TiO2 coupled with ruthenium-bipyridiyl complex and an electrolyte. It is possible for the solar cell to generate an electric current from the visible light through the ruthenium-bipyridiyl complex (i.e., photosensitive dye). However, the dye-sensitized solar cell should utilize a liquid electrolyte. Since a space is formed between glass or plastic plates by a spacer in the solar cell, and the solar cell is sealed, it is difficult to manufacture the solar cell, which increases manufacturing costs. Also, since a generated voltage is very low (about 0.7 volts), there are some limitations for commercializing the solar cell.
Another example of a dye-sensitized solar cell is included in Korean Unexamined Patent Publication No. 1997-7001433, in which a solar cell has a fiber structure and utilizes ZnO and molybdenum. Since the solar cell utilizes a glass fiber, it is necessary to coat a semiconductor and an electrode having a high melting point, thereby increasing costs.